feat(presets): pluggable preset system with template/command overrides, catalog, and resolver

- Rename 'template packs' to 'presets' to avoid naming collision with core templates
- PresetManifest, PresetRegistry, PresetManager, PresetCatalog, PresetResolver in presets.py
- Extract CommandRegistrar to agents.py as shared infrastructure
- CLI: specify preset list/add/remove/search/resolve/info
- CLI: specify preset catalog list/add/remove
- --preset option on specify init
- Priority-based preset stacking (--priority, lower = higher precedence)
- Command overrides registered into all detected agent directories (17+ agents)
- Extension command safety: skip registration if target extension not installed
- Multi-catalog support: env var, project config, user config, built-in defaults
- resolve_template() / Resolve-Template in bash/PowerShell scripts
- Self-test preset: overrides all 6 core templates + 1 command
- Scaffold with 4 examples: core/extension template and command overrides
- Preset catalog (catalog.json, catalog.community.json)
- Documentation: README.md, ARCHITECTURE.md, PUBLISHING.md
- 110 preset tests, 253 total tests passing

presets/self-test/preset.yml

@@ -0,0 +1,61 @@
+schema_version: "1.0"
+
+preset:
+  id: "self-test"
+  name: "Self-Test Preset"
+  version: "1.0.0"
+  description: "A preset that overrides all core templates for testing purposes"
+  author: "github"
+  repository: "https://github.com/github/spec-kit"
+  license: "MIT"
+
+requires:
+  speckit_version: ">=0.1.0"
+
+provides:
+  templates:
+    - type: "template"
+      name: "spec-template"
+      file: "templates/spec-template.md"
+      description: "Self-test spec template"
+      replaces: "spec-template"
+
+    - type: "template"
+      name: "plan-template"
+      file: "templates/plan-template.md"
+      description: "Self-test plan template"
+      replaces: "plan-template"
+
+    - type: "template"
+      name: "tasks-template"
+      file: "templates/tasks-template.md"
+      description: "Self-test tasks template"
+      replaces: "tasks-template"
+
+    - type: "template"
+      name: "checklist-template"
+      file: "templates/checklist-template.md"
+      description: "Self-test checklist template"
+      replaces: "checklist-template"
+
+    - type: "template"
+      name: "constitution-template"
+      file: "templates/constitution-template.md"
+      description: "Self-test constitution template"
+      replaces: "constitution-template"
+
+    - type: "template"
+      name: "agent-file-template"
+      file: "templates/agent-file-template.md"
+      description: "Self-test agent file template"
+      replaces: "agent-file-template"
+
+    - type: "command"
+      name: "speckit.specify"
+      file: "commands/speckit.specify.md"
+      description: "Self-test override of the specify command"
+      replaces: "speckit.specify"
+
+tags:
+  - "testing"
+  - "self-test"